Lemma 2 [1]} Suppose the characteristic polynomial \(p(\xi )\)  of Jacobian matrix (REF ) is given by \(p(\xi ) = \xi ^3 + a_1 \xi ^2 + a_2 \xi + a_3\) . Then the solutions \(\xi _i,~ i = 1, 2, 3,\)  of \(p(\xi ) = 0\)  satisfy \(\mid \xi _i\mid < 1\)  for \(i = 1, 2, 3\)  if the following conditions hold:
